Ground Zero 9/11 memorial flows with mournful splendour

How big should a memorial be? The fathomless horror of the first world war is remembered by the compact Cenotaph, but also by the aggregate of many cemeteries and monuments across Europe. The London Blitz has almost nothing. Maya Lin's memorial to the American casualties in Vietnam, in Washington DC (widely agreed to be one of the most eloquent and moving memorials of modern times) is – for its impact – relatively small.

In the case of September 11 2001, the answer is Very Big Indeed. The official memorial, now unveiled to the press in advance of the 10th anniversary of the attacks, has at its centre two of the largest fountains, or rather cascades, ever seen. Each occupies the exact footprint of one of the Twin Towers destroyed in the attacks and, as each tower was big, each cascade is a cuboid Niagara, an inverted eruption, falling 30 feet to a flat basin, and then another 30 feet through a smaller square hole in the centre.

Around the rim of each is a long bronze strip perforated with the names of victims: of the 2001 attacks on the Twin Towers and the Pentagon, of the hijack of Flight 93, which crashed in a field in Pennsylvania, and of the 1993 bombing of the World Trade Centre. The names, after years of agonising, are grouped by the location of each victim at the time of the attacks, modified by "adjacency requests" whereby relatives could ask for individual names to be by others to whom they were close.

The fountains stand in an eight-acre paved plaza, filled with 415 trees: they are all the same size, which required an exceptional effort of selecting and nurturing. The intention of the whole ensemble, say the architects Michael Arad and Peter Walker, is to make a place of both death and life – where victims can be properly remembered, but where office workers can come to eat their sandwiches. Underneath the plaza will be a large museum of the events, which is still under construction, and all around are rising office towers that will replace the 10m square feet of floor space that used to be on the site.

The cascades are simplified versions of Arad's original idea. He wanted the names to be at a lower level, reachable by walking behind the screens of falling water, but this proved too expensive and complex. What is there now impresses with its size, and makes reasonable decisions about the materials of commemoration – water, stone, trees, bronze – and how they might be deployed.

But memory should include detail as well as majesty, and it has to be said the memorial lacks intimacy. Usually, fountains – however big – allow you to dabble in them and sit at their edge. Here you can only gawp. You get the feeling that no one quite realised how big the cascades would be until it was too late, while the footprints of the towers might have been as potent if formed by a clearing in the trees, or by less drastic pools.

Walker is proud of the way his plaza catches rainwater to irrigate the trees. But this admirable detail seems a touch redundant next to the profligate gushing in its midst. The plan is that admission fees to the museum will help pay to keep the waters churning. It is unthinkable that they should stop, so – by whatever means and at whatever cost in money and energy – they will keep going for ever. It is impressive to contemplate this prospect, but also exhausting.